Figure 2  Schematic structures of a CoV and its spike protein. (a) The CoV is a pleomorphic spherical enveloped particle. It contains a linear positive-sense, single-stranded RNA with a 5′ cap and a 3′ poly(A) tail that are enclosed by nucleocapsid (N) proteins. The lipid bilayer envelope carries 3–4 structural proteins. (i) Membrane (M) proteins are the most abundant small triple-spanning transmembrane proteins that define the virion morphology and are major coordinators of virion assembly [140]. (ii) Envelope (E) proteins are minor channel-spanning transmembrane proteins that work together with the M proteins for virion assembly, budding and release [141]. (iii) Spike (S) proteins are homotrimeric type I transmembrane glycoproteins [142] protruding from the virion envelope (20 nm in length) that resemble a crown (Latin: corona) under an electron microscope. S proteins are major antigenic surface proteins and are critical for virion entry into a specific host cell by binding to a specific receptor(s) on the host cell surface and mediating membrane fusion [143]. (iv) The members of βCoV lineage A have additional short hemagglutinin esterase (HE) spikes (5 nm in length), which are homodimeric type I transmembrane glycoproteins. CoV HE has the potential to evolve its O-Ac-Sia receptor-binding specificity and activity, along with its companion S proteins, in balance with its receptor-destroying sialate O-acetylesterase activity for efficient virus infection and spread [86,94,144]. (b) Diagram of the unfolded S polypeptide composed of an N-terminal signal peptide (SP), an S1 subunit and an S2 subunit. The N-terminal signal peptide is a short type 1 signal hydrophobic peptide probably cleaved during cotranslational transport across the endoplasmic reticulum [145]. The S1 subunit carries 2 important domains, an S1 N-terminal domain (S1-NTD or S1A) and an S1 C-terminal domain (S1-CTD or S1B), one of which or both of which function as a host receptor-binding domain depending on the virus. Host receptors of CoVs affecting humans are shown above the bound S1 domain. The S1/S2 boundary with multiple basic aa residues may be potentially cleaved by furin during virus exit, while the monobasic cleavage site may be cleaved by a target cell protease, such as TMPRSS2 (trypsin-like protease) and cathepsin L [146], generating the S1 and S2 subunits with a noncovalently link [147]. The S2 subunit carries 5 regions. (i) The S2′ cleavage site may contain monobasic or two basic residues that must be cleaved by a host protease [146]. Both S1/S2 and S2′ cleavage sites must be cleaved to enable its S protein to mediate membrane fusion. (ii) A fusion peptide (FP) that mediates fusion of the virion envelope with the cellular plasma membrane or with the cellular endosomal membrane, followed by (iii) 2 heptad repeats (HR1 and HR2) promoting fusion [147], (iv) a transmembrane (TM) domain anchoring to the envelope and (v) a cytoplasmic tail (CT). (c) Side view of a surface diagram of a trimeric CoV S protein (pdb: 6q06 [148]). Each monomer of the S trimer is colored as follows: salmon, S1 and red, S2; marine blue, S1 and blue, S2; pale green, S1 and green, S2. S1-NTD and S1-CTD are on each S1 head above each S2 stalk.